The present invention relates to data communications and, more particularly, to techniques for measuring the performance of data communications systems.
A useful diagnostic parameter in such data communications systems as multipoint voiceband data networks is the so-called "missed start-up rate". This is the rate at which the central modem is unable--due to channel noise, gain or phase hits, etc.--to determine its start-up parameters (such as the receiver gain setting, the baud sampling phase and the carrier phase angle) with sufficient accuracy to enable error-free recovery of the subsequent data. A straightforward way of measuring this parameter is to have each tributary modem transmit ahead of the user-supplied data (hereinafter "user data") a header which includes (a) an address or other tributary-identifying indicium and (b) some mechanism for verifying the correctness of that address at the receiver, such as a checksum. If the checksum fails, a missed start-up is declared to have occurred.
A problem with this approach, however, is that if the checksum has failed, indicating that the header contains one or more errors, then the control modem cannot be assured that the tributary modem address contained in the header is correct. Thus although the control modem is able to determine that a missed start-up occurred, it will not be able to positively identify the particular tributary modem involved. As a practical matter, then, accurate measurement of the missed start-up rate on a pertributary-modem basis is not possible.
One way of providing a per-tributary-modem measurement of this parameter is to infer it via an observation of the dispersion of the signal points in the constellation of signal points received from each tributary modem. This approach is disadvantageous, however, in that the relationship between such dispersion and the actual missed start-up rate is approximate at best. A further disadvantage is that it requires significant real time signal processing capability.